1. Field of the Invention
The present invention relates to a method and apparatus for measuring an apparent density of a metal powder or a mixed powder containing a metal powder. In particular, the present invention relates to a method for measuring an apparent density of a metal powder in an on-line manner without stopping i) a process of producing the metal powder or a mixed powder containing the metal powder by mixing a raw material powder and auxiliary raw materials, which may be powders or liquids, with each other with a specified ratio using a mixer such that the metal powder or the mixed powder has a desired apparent density, or ii) a process of producing a powder compact. The present invention also relates to a method for producing a mixed powder having a desired apparent density using the above-described measurement method and a method for producing a powder compact.
2. Description of the Related Art
Powder metallurgy technology is a technology in which a metal powder serving as a raw material is filled in a die having a predetermined shape, compacted into a powder compact, and then sintered into a sintered compact. By further machining the sintered compact with a machining tool, when it is required, a machine part having a predetermined size and shape is produced. In the case where a high strength is required, a heat treatment such as carburizing or bright quenching may be performed after the sintering or the machining.
In recent years, with the advance of powder metallurgy technology, machine parts having complicated shapes have been produced in so-called near net shapes or in the shapes very close to their finished shapes with a high dimensional accuracy. Therefore, the machining can be simplified compared with that previously required or can be omitted and thus the production cost can be significantly reduced. Furthermore, a mixed powder produced by adding a graphite powder, an alloy powder, a lubricant, and the like to a metal powder in accordance with the intended purpose is used. Appropriate selection of types of powders serving as raw materials and various additives allow the production of machine parts having various characteristics suitable for the intended applications. With the above-described advance of technology, various machine parts produced using the powder metallurgy technology are used in various fields today.
The variations in the size, weight, and density of powder compacts before sintering cause variations in the size and characteristics of the sintered compacts, or the machine parts, that become finished products. Therefore, the variations in the size, weight, and density of powder compacts before sintering are preferably suppressed as much as possible to improve the quality of the machine parts. The variations in the size, weight, and density of the powder compacts mainly result from the variation in the filling density of the powder serving as a raw material filled in the die. To suppress the variation in the filling density, it is quite effective to stabilize the apparent density of the powder serving as a raw material. That is, the stability of the size and characteristics of the sintered compacts or the machine parts significantly depend on the stability of the apparent density of the powder serving as a raw material.
As described above, to produce high-quality machine parts having desired characteristics suitable for the intended applications and small variations in size and the like using powder metallurgy technology, it is quite important to stabilize the apparent density of the powder serving as a raw material.
Herein, the apparent density of a metal powder is generally measured by using the method specified in JIS Z 2504:2000. In this method, the apparent density of a powder is determined by filling the powder to be measured in a vessel having a specified volume under a specified condition, measuring the weight of the powder in the vessel, and dividing the measured weight of the powder by the volume of the vessel.
However, in this method for measuring the apparent density of a powder specified in JIS Z 2504:2000, the powder to be measured needs to be sampled. A powder in a mixer, hopper, or feeder can be subjected to the sampling measurement. In the sampling measurement, however, a mixing or a compacting operation needs to be temporarily stopped to perform the sampling. Accordingly, the apparent density cannot be continuously measured in an on-line manner.
Under such circumstances, for example, Japanese Unexamined Patent Application Publication No. 2005-199306 (Patent Document 1) discloses an apparatus for automatically measuring an apparent density of a metal powder for powder compacts. In the technology disclosed in Patent Document 1, a metal powder compacting apparatus including a feeder that holds a metal powder supplied from a powder supplying means and repeatedly performs an operation of moving the metal powder in the side direction and dropping in a cavity of a die to fill the die with the metal powder. The apparent density of the metal powder held in the feeder is calculated based on a capacitance of the metal powder held in the feeder measured by a powder capacitance measurement means. Patent Document 1 describes that this technology enables to quickly measure the apparent density of a metal powder for powder compacts without stopping fundamental steps of the powder compacting process.